Lesson 5

What is incomplete dominance?

In most games, there is a stronger team and a weaker team. Usually, the stronger team wins. Some games end in a tie. This shows that the teams are equally matched.

Heredity is sometimes like this. Most traits have a stronger, dominant gene, and a weaker, recessive gene.

The dominant gene usually “wins”. The dominant trait shows up in the offspring. The recessive trait stays hidden.

Not all genes are completely dominant or completely recessive. The genes of certain traits are equally strong. Neither trait is dominant.

We say there is incomplete dominance. In cases like these, genes combine, and mix.

A mixture of the two genes shows up. This kind of gene combination is called blending.

There are three good examples of incomplete dominance.

Four-o’clock Flowers

Four o’clock flowers are usually red or white. Red and white are equally strong traits. Neither color is dominant. When a pure red (RR) crosses with a pure white (WW), the colors blend.

The offspring will have pink (RW) flowers.

Shorthorn Cattle

In cattle, if one parent is pure red (RR) and the other parent is pure white (WW), the offspring will be pink (RW).

The blended calf is called a “roan”.

Andalusian Fowl

Some of these chickens have genes for black feathers, others have genes for white feathers. Neither of these genes is dominant. The offspring of pure black (BB) and pure white (WW) are gray (BW)

Many genes in humans also show incomplete dominance. They include genes for most hair and eye colors.